The major goal of this project is the definition of mechanisms of immune recognition of viral antigens on the cell surface of virus-infected cells. We are using cell lines persistently infected with murine leukemia viruses as our primary model system. FY7, a cloned variant of the (H-2b) murine leukemia FBL-3, has been found to express H-2Kb and H-2Db antigens recognized by antibody but not by T lymphocytes. Biochemical analysis has shown that these H-2 products of FY7 are expressed in an unaltered form. Properties of the cell membrane and perhaps the orientation of these antigens in the membrane of FY7 appear to determine the capacity of these cells to be recognized by T lymphocytes. This lack of CTL recognition can be reversed by passage of FY7 in vivo in the ascites form. This feature of FY7 is mimicked by the interaction of IgM but not IgG anti-H-2 antibody with this cell line. We are currently studying this phenomenon using monoclonal antibodies to cell membrane antigens of FY7 with the intention of quantifying antigenic sites on the cell membrane and the binding constant of the antigen-antibody interaction.